A new method for the selective precipitation of proteins is applied to the isolation and purification of an antibody. Ligand-modified phospholipids (LMPs) are solubilized by the nonionic ethoxylated alcohol detergent, resulting in small (50 to 100 Angstrom) micellar aggregates of LMPs and surfactant. When introduced into protein solutions containing an antibody for which the LMP has specific affinity, the ligand binds to the protein. Hydrophobic interactions between phospholipid tail groups bound to the protein molecules result in an insoluble precipitate. Polyclonal and monoclonal antibiotin antibody (pABA and mABA) are shown to be selectively precipitated using ratios of dimyristoylphosphatidylethanolamidobiotin (DMPE-B) to ABA ranging from 1:1 to 19:1. The kinetics and yield of the precipitation achieve a maximum at a ratio of DMPE-B to ABA of approximately 7:1. The kinetics and magnitude of the turbidity change are modeled using the Mie theory of light scattering coupled with the Smoluchowski theory of aggregation. The kinetics are shown to be enhanced significantly by the addition of salt. In particular, the addition of 0.5 M ammonium sulfate salt increases the rate of precipitation by more than an order of magnitude. It is demonstrated that pABA can be recovered with total activity yields of 60% to 70% from mixtures containing nonspecific IgG antibodies in very high purity (> 99%).